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<title>Filter Iterator Adaptor Documentation</title>
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<img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)"
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align="center" width="277" height="86">
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<h1>Filter Iterator Adaptor</h1>
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Defined in header
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<a href="../../boost/iterator_adaptors.hpp">boost/iterator_adaptors.hpp</a>
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<p>
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The filter iterator adaptor creates a view of an iterator range in
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which some elements of the range are skipped over. A <a
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href="http://www.sgi.com/tech/stl/Predicate.html">Predicate</a>
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function object controls which elements are skipped. When the
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predicate is applied to an element, if it returns <tt>true</tt> then
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the element is retained and if it returns <tt>false</tt> then the
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element is skipped over.
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<h2>Synopsis</h2>
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<pre>
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namespace boost {
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template <class Predicate, class Iterator, ...>
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class filter_iterator_generator;
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template <class Predicate, class Iterator>
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typename filter_iterator_generator<Predicate, Iterator>::type
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make_filter_iterator(Iterator first, Iterator last, const Predicate& p = Predicate());
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}
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</pre>
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<h2><a name="filter_iterator_generator">The Filter Iterator Type
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Generator</a></h2>
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The class <tt>filter_iterator_generator</tt> is a helper class who's
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purpose is to construct a filter iterator adaptor type. The template
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parameters for this class are the <tt>Predicate</tt> function object
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type and the <tt>Iterator</tt> type that is being wrapped. In most
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cases the associated types for the wrapped iterator can be deduced
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from <tt>std::iterator_traits</tt>, but in some situations the user
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may want to override these types, so there are also template
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parameters for each of the iterator's associated types.
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<pre>
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template <class Predicate,
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class Iterator,
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class Value = std::iterator_traits<Iterator>::value_type,
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class Pointer = std::iterator_traits<Iterator>::pointer,
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class Reference = std::iterator_traits<Iterator>::reference,
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class Category = std::iterator_traits<Iterator>::iterator_category,
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class Distance = std::iterator_traits<Iterator>::difference_type,
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class filter_iterator_generator
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{
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public:
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typedef ... type; // the filter <a href="./iterator_adaptor.htm"><tt>iterator_adaptor</tt></a> type
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typedef ... policies_type; // the filter policies type
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}
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</pre>
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<h3>Example</h3>
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The following example uses filter iterator to print out all the
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positive integers in an array.
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<pre>
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struct is_positive_number {
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bool operator()(int x) { return 0 < x; }
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};
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int main() {
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int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
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const int N = sizeof(numbers)/sizeof(int);
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typedef boost::filter_iterator_generator<is_positive_number, int*, int> Gen;
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is_positive_number predicate;
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Gen::policies_type policies(numbers + N, predicate);
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Gen::type filter_iter_first(numbers, policies);
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Gen::type filter_iter_last(numbers + N, policies);
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std::copy(filter_iter_first, filter_iter_last, std::ostream_iterator<int>(std::cout, " "));
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std::cout << std::endl;
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return 0;
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}
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</pre>
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The output is:
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<pre>
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4 5 8
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</pre>
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<h3>Template Parameters</h3>
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<Table border>
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<TR>
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<TH>Parameter</TH><TH>Description</TH>
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</TR>
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<TR>
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<TD><a href="http://www.sgi.com/tech/stl/Predicate.html"><tt>Predicate</tt></a></TD>
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<TD>The function object that determines which elements are retained and which elements are skipped.
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</TR>
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<TR>
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<TD><tt>Iterator</tt></TD>
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<TD>The iterator type being wrapped. This type must at least be a model
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of the <a href="http://www.sgi.com/tech/stl/InputIterator">InputIterator</a> concept.</TD>
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</TR>
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<TR>
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<TD><tt>Value</tt></TD>
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<TD>The <tt>value_type</tt> for the iterator adaptor. Typically the default for
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this parameter is the appropriate type<a href="#1">[1]</a>.<br>
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<b>Default:</b> <tt>std::iterator_traits<Iterator>::value_type</TD>
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</TR>
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<TR>
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<TD><tt>Pointer</tt></TD>
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<TD>The <tt>pointer</tt> type for the iterator adaptor. Typically the default for
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this parameter is the appropriate type.<br>
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<b>Default:</b> <tt>std::iterator_traits<Iterator>::pointer</TD>
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</TR>
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<TR>
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<TD><tt>Reference</tt></TD>
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<TD>The <tt>reference</tt> type for the iterator adaptor. Typically the default for
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this parameter is the appropriate type.<br>
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<b>Default:</b> <tt>std::iterator_traits<Iterator>::reference</TD>
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</TR>
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<TR>
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<TD><tt>Category</tt></TD>
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<TD>The <tt>iterator_category</tt> type for the iterator adaptor.
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Typically the
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default for this parameter is the appropriate type. If you override
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this parameter, do not use <tt>bidirectional_iterator_tag</tt>
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because filter iterators can not go in reverse.<br>
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<b>Default:</b> <tt>std::iterator_traits<Iterator>::iterator_category</TD>
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</TR>
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<TR>
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<TD><tt>Distance</tt></TD>
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<TD>The <tt>difference_type</tt> for the iterator adaptor. Typically the default for
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this parameter is the appropriate type.<br>
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<b>Default:</b> <tt>std::iterator_traits<Iterator>::difference_type</TD>
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</TR>
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</table>
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<h3>Model of</h3>
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The filter iterator adaptor (the type
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<tt>filter_iterator_generator<...>::type</tt>) may be a model of <a
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href="www.sgi.com/tech/stl/InputIterator.html">InputIterator</a> or <a
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href="www.sgi.com/tech/stl/ForwardIterator.html">ForwardIterator</a>
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depending on the adapted iterator type.
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<p>
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<hr>
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<p>
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<h2><a name="make_filter_iterator">The Make Filter Iterator Function</a></h2>
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<pre>
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template <class Predicate, class Iterator>
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typename detail::filter_generator<Predicate, Iterator>::type
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make_filter_iterator(Iterator first, Iterator last, const Predicate& p = Predicate())
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</pre>
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This function provides a convenient way to create filter iterators.
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<h3>Example</h3>
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In this example we print the positive number again, this time using
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the <tt>make_filter_iterator()</tt> function.
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<pre>
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struct is_positive_number {
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bool operator()(int x) { return 0 < x; }
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};
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int main()
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{
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int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
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const int N = sizeof(numbers)/sizeof(int);
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std::copy(boost::make_filter_iterator<is_positive_number>(numbers, numbers + N),
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boost::make_filter_iterator<is_positive_number>(numbers + N, numbers + N),
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std::ostream_iterator<int>(std::cout, " "));
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std::cout << std::endl;
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return 0;
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}
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</pre>
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The output is:
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<pre>
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4 5 8
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</pre>
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<h3>Notes</h3>
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<a name="1">[1]</a> If the compiler does not support partial
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specialization and the wrapped iterator type is a builtin pointer then
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the <tt>Value</tt> type must be explicitly specified (don't use the
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default).
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<hr>
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<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->10 Feb 2001<!--webbot bot="Timestamp" endspan i-checksum="14373" --></p>
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<p>© Copyright Jeremy Siek 2000. Permission to copy, use,
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modify, sell and distribute this document is granted provided this copyright
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notice appears in all copies. This document is provided "as is"
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without express or implied warranty, and with no claim as to its suitability for
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any purpose.</p>
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